Cosmic Background Radiation

If we look around us with the right instruments, we see CBR all around and it is fundamentally isotropic, that means on average the temperature and frequency is the same no matter which direction we look. (Doesn't it?)

Say we are passed by someone travelling at 0.6c. At the moment that traveller passes us, they should receive (on average) the same sort of photons that we do. But should they not see them differently due to their relative speed?

This sounds to me like an indicator for a frame which has some sort of absolute motion. If you look around you and see the CBR is red shifted in one direction and blue shifted in the opposite direction, then would that not indicate that you are not at rest?

I've been scratching my head to work out why this wouldn't work (because I am pretty sure it wouldn't, I just can't currently see why).

Staff: Mentor

If we look around us with the right instruments, we see CBR all around and it is fundamentally isotropic, that means on average the temperature and frequency is the same no matter which direction we look. (Doesn't it?)

Not quite. There's a small anisotropy which is attributed to the motion of the solar system around the center of our galaxy. See the U2 Anisotropy Experiment.

Say we are passed by someone travelling at 0.6c. At the moment that traveller passes us, they should receive (on average) the same sort of photons that we do. But should they not see them differently due to their relative speed?

Yes.

This sounds to me like an indicator for a frame which has some sort of absolute motion.

When we talk about "absolute motion" it's usually in the context of assuming a special (inertial) reference frame in which the laws of physics take a special, particularly simple form. The laws of physics are the same in the CMB rest frame as in the earth's rest frame, as far as I know (ignoring of course any slight non-inertialness of the earth's motion). Observations of specific physical quantities vary from one frame to another, of course, but the laws that relate those quantities (conservation of energy, conservation of momentum, etc.) are the same.

One way to re-phrase this is: without "looking" at the CMB itself, it is (as far as I know) impossible to decide whether we are moving with respect to the CMB. This is similar to the (hopefully well-known) fact that if we are in a train moving with constant speed along a perfectly straight, level, smooth track, with silent engines, and with shuttered windows, we cannot tell (without being able to "look outside") whether we are moving or not.

If we look around us with the right instruments, we see CBR all around and it is fundamentally isotropic, that means on average the temperature and frequency is the same no matter which direction we look. (Doesn't it?)

Say we are passed by someone travelling at 0.6c. At the moment that traveller passes us, they should receive (on average) the same sort of photons that we do. But should they not see them differently due to their relative speed?

This sounds to me like an indicator for a frame which has some sort of absolute motion. If you look around you and see the CBR is red shifted in one direction and blue shifted in the opposite direction, then would that not indicate that you are not at rest?

Yes, in some sense, the cosmic microwave backgrond (CMB) does the provide an absolute cosmological frame, and our solar system is not at rest with respect to this frame. For us, the CMB is blueshifted in one direction and redshifted in the other direction. Images of the CMB often are images from which the solar's sytems relative motion has been subtracted.

Yes, like all the other posts are saying, the CMB has a rest frame and the earth is moving with respect to this frame. But that doesn't really qualify as an "absolute inertial frame", but rather just as a "special" one. The same goes for the center of mass-frame of our milky way, or our solar system. They are all "special" inertial frames, but none of them should be considered as absolute. Relativity principles forbid the existence of such frames.

This sounds to me like an indicator for a frame which has some sort of absolute motion. If you look around you and see the CBR is red shifted in one direction and blue shifted in the opposite direction, then would that not indicate that you are not at rest?

Yes, definitely. Ordinary workaday cosmology is based on that idea of rest.
The Hubble Law is explicitly about the distance now between two widely separated observers both of which are at rest with respect to the matter of the early universe and the bath of light that everybody receives from it.

You cannot say now without a criterion of rest.

Before the CMB was observed, Hubble observed the socalled hubble flow, which gives a similar criterion of rest. If you are moving relative to hubble flow then ahead of you the galaxies will disobey the Law and will seem to recede more slowly, in proportion to their distances, than the law requires, and the galaxies behind you will seem to recede more rapidly.

The solar system is moving at about 370 km/s relative to hubble flow, and in the direction marked by constellation Leo.
Now we do not use hubble flow as a measure of rest and absolute motion because we have CMB. It is the same criterion, just more accurate.

Observations of CMB are always adjusted to correct for the Doppler dipole, to get rid of the 370 km/s absolute motion.

This is routine in cosmology.

So your intuition was quite correct. You just reproduced in your own mind what the professionals have already been doing for many years.

One way to re-phrase this is: without "looking" at the CMB itself, it is (as far as I know) impossible to decide whether we are moving with respect to the CMB. This is similar to the (hopefully well-known) fact that if we are in a train moving with constant speed along a perfectly straight, level, smooth track, with silent engines, and with shuttered windows, we cannot tell (without being able to "look outside") whether we are moving or not.

Naturally I know about the train, and the nature of inertial motion.

The difference here is that there is no "outside" to look at, is there? There's no outside the CMB.

What is bothering me is that if you go out to interstellar (or better intergalactic) space, it sounds like you can find an inertial trajectory (v0) in which there is no red/blue shifting of the CMB. Why is this not "at rest relative to the universe"?

If it is, then, using inertial gyros, you should be able to track the changes made to match the inertial motion any other body and thereby provide a speed relative to v0, which is therefore a speed relative to the universe. A problem here is that you can accelerate forever, which would give you ridiculous speeds (but sensible rapidities), but since you are only manoeuvering to match other inertial motions, I doubt it would be necessary to accelerate forever.

Once you have matched the inertial motion of another body, since you know what changes you made (by means of inertial gyros/accelerometers), should you not then be able to say what sort of red/blue shift that other body should experience when viewing the CMB? That should be testable, shouldn't it?

Or, if one observer went to intergalactic space on the other side of the Milky Way and one to intergalactic space on this side, would they both find different speeds at which the CMB has no red/blue shift (ie after finding themselves at rest with respect to the CMB, they would then find that they are not at rest with respect to each other)? Alternatively, if one observer went out to intergalactic space found and maintained a (safe) trajectory which was at rest with respect to the CMB and went into cryonic suspension for a billion years, would that observer on revival still find that the trajectory is at rest with respect to the CMB?

What I am getting at is there is on one hand the statement "Relativity principles forbid the existence of (absolute) frames" from xepma and most of those interested in relativity, and on the other hand "in some sense, the cosmic microwave backgrond (CMB) does the provide an absolute cosmological frame" from George Jones (who gets extra points for being an expert in the area).

Is this reconciled by considering jtbell's comment "When we talk about "absolute motion" it's usually in the context of assuming a special (inertial) reference frame in which the laws of physics take a special, particularly simple form" - in other words, are we permitted to have an "absolute" rest frame (meaning at rest with respect to the universe) so long as that is not meant to imply that there are special forms of the laws of physics applicable in that frame? Or, is the "absolute" rest frame absolutely forbidden?

(I think that if the intergalactic observers found that they could both be at rest with respect to CMB while not being at rest with respect to each other or at rest at one time and later not at rest even while maintaining the same inertial trajectory, then that would imply that an "absolute" rest frame is forbidden not just by scholars, but also the universe.)

The difference here is that there is no "outside" to look at, is there? There's no outside the CMB.

What is bothering me is that if you go out to interstellar (or better intergalactic) space, it sounds like you can find an inertial trajectory (v0) in which there is no red/blue shifting of the CMB. Why is this not "at rest relative to the universe"?

If it is, then, using inertial gyros, you should be able to track the changes made to match the inertial motion any other body and thereby provide a speed relative to v0, which is therefore a speed relative to the universe. A problem here is that you can accelerate forever, which would give you ridiculous speeds (but sensible rapidities), but since you are only manoeuvering to match other inertial motions, I doubt it would be necessary to accelerate forever.

Once you have matched the inertial motion of another body, since you know what changes you made (by means of inertial gyros/accelerometers), should you not then be able to say what sort of red/blue shift that other body should experience when viewing the CMB? That should be testable, shouldn't it?

It's called the kinetic Sunyaev-Zel'dovich effect. In principle detectable, but not easy to do so.

Or, if one observer went to intergalactic space on the other side of the Milky Way and one to intergalactic space on this side, would they both find different speeds at which the CMB has no red/blue shift (ie after finding themselves at rest with respect to the CMB, they would then find that they are not at rest with respect to each other)? Alternatively, if one observer went out to intergalactic space found and maintained a (safe) trajectory which was at rest with respect to the CMB and went into cryonic suspension for a billion years, would that observer on revival still find that the trajectory is at rest with respect to the CMB?

Observers separated by some distance that entered their rest frames with respect to the CMB would discover that they were also moving apart from one another by an amount proportional to their distance (due to the expansion of the universe).

What I am getting at is there is on one hand the statement "Relativity principles forbid the existence of (absolute) frames" from xepma and most of those interested in relativity, and on the other hand "in some sense, the cosmic microwave backgrond (CMB) does the provide an absolute cosmological frame" from George Jones (who gets extra points for being an expert in the area).

Is this reconciled by considering jtbell's comment "When we talk about "absolute motion" it's usually in the context of assuming a special (inertial) reference frame in which the laws of physics take a special, particularly simple form" - in other words, are we permitted to have an "absolute" rest frame (meaning at rest with respect to the universe) so long as that is not meant to imply that there are special forms of the laws of physics applicable in that frame? Or, is the "absolute" rest frame absolutely forbidden?

The CMB isn't an "absolute" frame, though. It's just one frame of reference.

Observers separated by some distance that entered their rest frames with respect to the CMB would discover that they were also moving apart from one another by an amount proportional to their distance (due to the expansion of the universe).

But the cryonically stored observer will still be at rest with respect to the CMB upon waking, yes?

I'm aware that there will be some cosmological expansion fueled separation between the spatial separated observers, but that wasn't quite what I was thinking about. I was thinking more about the fact that some photons from the CMB pass observer A, then some time later pass observer B, while other photons pass B first and then pass observer A.

Photons travelling along the axis joining A and B could pass one of them then the other. A should expect that photons which pass A first and then B will be red shifted according to B while B should expect that photons which pass B first and then A will be red shifted according to A. Similarly, both will think that when the other sees a photon first, it will be blue shifted. See the diagram for A.

There must be something wrong with the way I am thinking about this, because both A and B should see CMB with the same colour from all directions, right? (Recall that in the scenario A and B specifically set themselves up to be at rest with respect to the CMB.)

Viscerally, I suspect that spatially separated observers might have to be at rest with respect to each other (in such a way as to negate the universal expansion speeds) in order to be at rest with respect to the CMB.

Attached Files:

But the cryonically stored observer will still be at rest with respect to the CMB upon waking, yes?

If the universe were perfectly smooth, then this would be the case. However, it is not: any observer at rest with respect to the CMB would be likely to find themself with a large mass somewhat closer in one direction than another, and would therefore tend to fall towards that mass.

I'm aware that there will be some cosmological expansion fueled separation between the spatial separated observers, but that wasn't quite what I was thinking about. I was thinking more about the fact that some photons from the CMB pass observer A, then some time later pass observer B, while other photons pass B first and then pass observer A.

Right. A good way to think about this situation is that if we take this CMB reference frame to determine a global time coordinate, then we can start to talk about how much time it takes for a photon to pass from observer A to observer B (or vice versa). Obviously, it will take some amount of time. And in that time the universe will have expanded by some amount. It turns out that the photons are expanded by the exact same amount, and so the wavelength increases the longer the photon travels. It doesn't matter which direction it travels in, just that it travels in some direction for some amount of time.

There must be something wrong with the way I am thinking about this, because both A and B should see CMB with the same colour from all directions, right? (Recall that in the scenario A and B specifically set themselves up to be at rest with respect to the CMB.)

Yes, but if we're thinking a photon first passing by observer A, then that photon will be observed at an earlier time than the same photon observed by observer B. Observer B will see all CMB photons from all directions at this longer wavelength, because the universe has expanded while we waited for this one particular photon passing from observer A to reach B.

Yes, but if we're thinking a photon first passing by observer A, then that photon will be observed at an earlier time than the same photon observed by observer B. Observer B will see all CMB photons from all directions at this longer wavelength, because the universe has expanded while we waited for this one particular photon passing from observer A to reach B.

We picked the locations of A and B at random (or at least I did, I didn't say which was where, nor even specified where "where" was) so how can we possibly say that B receives all CMB photons from all directions at a longer wavelength than A does?

Once we take into account the simultaneity issues (since A and B are not at rest with respect to each other due to cosmological expansion), I would imagine that the randomly located A and B would have to receive the same sort of photons, at the same sort of colour, with the same sort of distribution.

We picked the locations of A and B at random (or at least I did, I didn't say which was where, nor even specified where "where" was) so how can we possibly say that B receives all CMB photons from all directions at a longer wavelength than A does?

Because we're talking about a later time. The universe has expanded in the interim.

Once we take into account the simultaneity issues (since A and B are not at rest with respect to each other due to cosmological expansion), I would imagine that the randomly located A and B would have to receive the same sort of photons, at the same sort of colour, with the same sort of distribution.

Right, there is an arbitrariness in the time coordinate, but as I said, you can just set the time coordinate based upon the expansion of the universe, in the case measured via the CMB. In this case the time coordinate that we're using is the proper time of a clock at rest with respect to the CMB at that particular location, starting at some "equal-time slicing" of the universe where the universe was at equal density when the various clocks read the same value. Neglecting density fluctuations for the time being, such clocks will always read the same time value when they see the same density and temperature of the universe.

Because we're talking about a later time. The universe has expanded in the interim.

Right, there is an arbitrariness in the time coordinate, but as I said, you can just set the time coordinate based upon the expansion of the universe, in the case measured via the CMB. In this case the time coordinate that we're using is the proper time of a clock at rest with respect to the CMB at that particular location, starting at some "equal-time slicing" of the universe where the universe was at equal density when the various clocks read the same value. Neglecting density fluctuations for the time being, such clocks will always read the same time value when they see the same density and temperature of the universe.

I should have read more closely, shouldn't I?

On a cosmological scale, are you saying that a particular rest colour of the CMB represents a moment of simultaneity throughout the universe? - where a "rest colour" is the spectrum of the CMB for an observer who is at rest with the CMB as previously discussed.

Is there anything preventing us from using this as our universal clock (assuming that we could overcome the inaccuracy of measurement)?

The scheme I am thinking of here is this: check that you are inertial (absence of forces), check the CMB in all directions, note the axis of your motion (point of maximum red-shift to point of maximum blue-shift) and then note the colour of the CMB from a direction perpendicular to the axis of motion, take into acount your motion (from the degree of red-/blue-shift) and then you can get the resting CMB colour. Then you know "when" you are in a pretty much absolute sense.

Wouldn't you?

I stress that it is not a question of accuracy here, I am more interested in whether you can talk about a universal "when" even if you can't pin it down to the precise year, or even millenium.

On a cosmological scale, are you saying that a particular rest colour of the CMB represents a moment of simultaneity throughout the universe? - where a "rest colour" is the spectrum of the CMB for an observer who is at rest with the CMB as previously discussed.

Is there anything preventing us from using this as our universal clock (assuming that we could overcome the inaccuracy of measurement)?

No, not really. In fact, this is precisely the time coordinate used for most of cosmological studies. But it is worth mentioning that it's still an arbitrary choice. It's just nice to use because it's something that's observable globally, and very easy to decide what "simultaneous" means, even if it is an arbitrary choice.

No, not really. In fact, this is precisely the time coordinate used for most of cosmological studies. But it is worth mentioning that it's still an arbitrary choice. It's just nice to use because it's something that's observable globally, and very easy to decide what "simultaneous" means, even if it is an arbitrary choice.

I'd just like to work out just what you mean when you say "it's still an arbitrary choice".

Do you mean arbitrary in the same sense that if I were to refer to the ground over which I drive my car as stationary is an arbitrary choice?

In other words, thinking of the Earth as stationary is arbitrary but very sensible.

I'd just like to work out just what you mean when you say "it's still an arbitrary choice".

Do you mean arbitrary in the same sense that if I were to refer to the ground over which I drive my car as stationary is an arbitrary choice?

It's arbitrary in the sense that one could make any number of other possible choices of reference frame and the laws of physics would still be exactly the same. That is to say, the fact remains that you can't tell whether or not you're moving with respect to the CMB without looking at the CMB.

It's arbitrary in the sense that one could make any number of other possible choices of reference frame and the laws of physics would still be exactly the same. That is to say, the fact remains that you can't tell whether or not you're moving with respect to the CMB without looking at the CMB.

So the same as jtbell's scenario -

jtbell said:

One way to re-phrase this is: without "looking" at the CMB itself, it is (as far as I know) impossible to decide whether we are moving with respect to the CMB. This is similar to the (hopefully well-known) fact that if we are in a train moving with constant speed along a perfectly straight, level, smooth track, with silent engines, and with shuttered windows, we cannot tell (without being able to "look outside") whether we are moving or not.

In other words, thinking of the Earth as stationary is arbitrary but very sensible.
...

I like that way of putting it!

Arbitrary but very sensible.

Just as a quibble, knowing what we do about the universe, we can tell our speed relative to CMB without looking at the CMB.

We can look at other markers to see how we are moving relative to the average matter of the early universe. We can tell the same speed by looking at galaxies' redshifts.

Because we are moving 370 km/s in the direction of Leo relative to bulk, the galaxies in that part of the sky are receding on average slower (by 370 km/s) than Hubble Law says they should. And the galaxies behind us, opposite to Leo direction, are receding on average faster (by 370 km/s) than Law says to expect.

The Hubble law about galaxy recession/distance does not work, is not true, unless you correct for the motion of the solar system relative to ancient matter, or the CMB light from it, or whatever you like to call it.

Of course it is arbitrary!

But it is very convenient so cosmologists use that criterion of rest and associated simultaneity, for nearly everything they do.

If you move too fast in some direction you will be roasted by the CMB doppler hotspot in that direction. So, like you said, being at rest relative CMB is "very sensible".

Just as a quibble, knowing what we do about the universe, we can tell our speed relative to CMB without looking at the CMB.

We can look at other markers to see how we are moving relative to the average matter of the early universe. We can tell the same speed by looking at galaxies' redshifts.

Well, that's only because we know these other things also have the same reference frame, at least on average. One might argue that if we didn't know this already through observation, we may not be able to use the galaxies' motion as a means to determine the rest frame of the CMB.